This invention relates to a towing mechanism for connecting a towed vehicle to a towing vehicle and, more particularly, to an improved steering assembly for towable vehicles having steerable front wheels. Without any intention of restricting the broad scope of the invention, the towed vehicle will sometimes be referred to as a trailer and the towing vehicle as an automobile, the invention being especially useful for a combination of such vehicles.
It is well recognized that when a towed vehicle, such as a travel trailer, is towed behind a towing vehicle, such as a car or truck, there is a tendency of the towed vehicle to sway when traveling at highway speeds, thus causing a hazardous or dangerous driving condition. This condition may be aggravated by uneven roadbeds, sudden turns by the towing vehicle, and air buffeting caused by gusty winds or large passing vehicles traveling at high speeds. Prior to the present invention, the inventor therefore investigated the stability of many different types of towed vehicles.
Three types or "degrees" of rotational movement are needed between the towing and towed vehicles for the combination to travel over uneven road surfaces and execute turns. The three degrees of movement are called pitch, roll and yaw and correspond to the three axes about which relative movements are permitted between the towing and towed vehicles, namely, rotation about a horizontal lateral axis (pitch), about a horizontal longitudinal axis (roll) and about a vertical pivot axis (yaw). While the direction of these axes may vary to some extent within the design and mechanical tolerances employed in towing devices, the yaw axis is substantially vertical and the pitch and roll axes are substantially horizontal.
Conventional two wheel, center-axle trailers usually use a trailer hitch comprised of a single tongue or tow bar rigidly connected at one end to the forward frame of the towed vehicle and detachably connected at the other end to the towing vehicle by a ball joint. The ball joint is usually mounted on a frame supported member at or near the rear bumper of the towing vehicle. This ball joint connection provides, at least to a limited extent, the three degrees of rotational movement referred to above. Thus, the effective pivot point between the towing vehicle and a center-axle trailer with a conventional hitch is at the ball joint, which is usually located a substantial distance behind the rear wheels of the towing vehicle. As a result, a large lever arm exists between the pivotal connection and the center of gravity of the towing vehicle. Consequently, a significant force is applied to the towing vehicle by the towed vehicle when the vehicles are rounding a curve which tends to displace or sway the towing vehicle to one side. This upsetting force may also occur with other changes in angle between the longitudinal axes of the two vehicles during forward surge of the trailer while traveling down a steep hill or braking. Control over both vehicles may thus be reduced and in some cases lost.
Center-axle trailers are usually manageable as long as the ratio of trailer to automobile weight is favorable. The automobile ideally should be heavier than such trailers by a ratio of 2 to 1. As the trailer to automobile weight ratio becomes equal, or the trailer becomes heavier than the automobile, the dynamic forces of the trailer begin to override the inherent stability of the towing vehicle. Although there have been many attempts in the prior art to increase the stability of this combination, there are unfortunately many unsuitable devices and combinations being used as evidence by the high accident rate among automobiles and other relatively light vehicles while towing heavy trailers. Such prior art devices are just not able to provide the needed anti-sway control at highway speeds and permit dynamic oscillations of the loading of the trailer and of the towing vehicle to become synchronized in an adverse phase relationship. When this happens, the forces of instability multiply rapidly and are often greater than either the device itself or the driver of the towing vehicle is able to control. In fact, the efforts of even the most experienced driver under these conditions often aggravate the situation by contributing to the adverse phase relationship.
As speed increases, only small changes in steering angle or in the angle between towing and towed vehicles are required to produce large upset forces. Above a certain speed or crossover point, dynamic oscillations in an adverse phase relationship can increase those forces to a level that cannot be controlled. Once the automobile and the trailer are traveling beyond the safe speed for which directional control can be maintained by the driver, it takes very little to set such an adverse oscillation into motion. Dynamic instability may be initiated by gusty cross winds, changes in the road surface, rapid lane changes, curves in the roadway, or a flat tire or blow out.
Instability of the combination is often aggravated by improper loading of the trailer which can shift its center of gravity to an even more adverse location and bring on uncontrollable oscillations at an unsuspectingly low speed. With center-axle trailers, the amount of tongue weight applied to the rear of the towing vehicle is critical to the stability of the combination. This requirement of a particular tongue weight puts considerble constraints on the manner of loading such towed vehicles.
The disadvantages of the center-axle trailer may be overcome in part by employing a four wheel trailer with separate or "split" axles, one near the front and the other near the rear of the trailer. This arrangement has the advantage that the front axle of the trailer carries substantially all of the weight of the front of the trailer such that considerably less attention need be given to weight distribution inside the trailer. The four wheel trailer also provides a more stable unit since a four point suspension is utilized as opposed to a three point suspension, one of which is the rear of the towing vehicle. Lever arm forces are also reduced with the four wheel trailer which includes front wheels that can be steered to follow the towing vehicle.
A towable split-axle trailer conventionally includes a tow bar or the like with one end connected to the towing vehicle and the other end connected to steering apparatus operable to impart steering movements to the trailer's steerable front wheels. One conventional form of steering mechanism for such trailers is known as the "Ackerman" steering system. The term "Ackerman" as used in this specification refers to the automotive type system utilizing steering spindles or knuckles pivotally mounted upon king pins carried by a frame or axletree and connected by bell crank and tie rod linkages. A single wheel on each side has an individual axle mounted upon a corresponding knuckle and steerable independently of the axletree. This system does not permit the wheels to have a turning angle of more than about 45 degrees. Sometimes the towing vehicle must make an exceptionally sharp turn beyond the steering capability of the Ackerman system and it is not uncommon in such instances for the steering mechanism to be broken or subjected to severe damage.
One way in which the prior art obtains a greater degree of turning maneuverability for split-axle trailers is to provide the trailer with a fifth wheel type of turning mechanism. These trailers are guided like a farm wagon where the entire front axle and wheel assembly is mounted upon a lubricated, rotatable plate resembling a fifth vehicle wheel so that the angle of the axle changes relative to the longitudinal axis of the trailer. The trailer is guided by moving the front axle so as to change its angle with respect to the longitudinal axis of the trailer. This is referred to as axle steering and the change in angle produces what is referred to herein as angle steering. This construction provides a stable trailer with steerable wheels that can be turned over 90 degrees to either side of center. However, the steerable wheels are unable to move independently and this can cause tire scuffing when traveling over uneven road surfaces. Although the stability of these trailers in forward tow is usually guite good, unstable oscillations may develop if the vertical pivot connections are loose or the fifth wheel does not pivot freely due to improper lubrication. Trailers of this type also have a marked tendency to jackknife under surge forces resulting from braking or downhill travel of the towing vehicle.
Many conventional trailer steering devices are also deficient in that the trailer will either go wide of the path of the rear wheels of the towing vehicle or cut inside of that path on turns. Such movements are undesirable in that they cause scuffing of either the automobile or the trailer tires, depending upon which vehicle has the greater weight. Such action can also lead to unstable oscillatory motions between the two vehicles at highway speeds. It is therefore desirable that the trailer wheels closely track the rear wheels of the towing vehicle in forward travel without imposing transverse forces on the rear of the towing vehicle.
Another prior art device is the semi-trailer rig where the trailer is attached over the rear wheels of the tractor and uses those wheels for its front wheels. This unit is inherently stable, meaning that the dynamic forces of the trailer do not upset the inherent stability of the towing vehicle. In the forward direction, the semi-trailer will obviously follow the tractor because the rear tractor wheels form the front guiding or steering wheels of the trailer. Similarly, upon backing, all steering is accomplished by an angle change between the tractor and the trailer such that the rig can easily be backed by the driver steering the tractor for the proper angle change of the tractor's rear wheels to steer the trailer in the desired turning direction. With the vertical pivot axis at or near the center of the rear axle, the dynamic forces of the trailer under most conditions, except skidding of the tractor as discussed below, have practically no adverse effect on the directional stability of the tractor. Because of this stability and its complete freedom of maneuverability, the tractor semi-trailer rig has become the standard of the commercial trucking industry and enjoys the reputation of being the most stable and manageable of all methods of hauling a towed vehicle outweighing the towing vehicle by several times.
It has thus been recognized that the stability and backing control of center-axle trailers are improved by attaching the trailer to a point over or slightly forward of the driving rear axle of the towing vehicle by a fifth wheel connection similar to that used in axle steering. In this system, the towing vehicle must be specially modified to carry the fifth wheel in a manner similar to the tractor of the semi-trailer rig. Such trailers have been connected to the bed of pickup trucks at a position either above or slightly forward of the rear axle by means of a fifth wheel connection known as a "gooseneck".
When connecting a trailer to an automobile or other fully enclosed motor vehicle, the space above or in front of the rear axle is often unavailable or inconvenient for attachment of a trailer. While a fifth wheel therefore provides an improved measure of stability and handling, its utility is limited to specially designed towing vehicles. The convenience of connecting and disconnecting a towed vehicle from the rear end of a motor vehicle has lead to the almost universal attachment of trailer hitches at or near the rear bumper of automobiles. Furthermore, while the fifth wheel connection is relatively stable during towing, it has a tendency to jackknife during braking.
It is also known in the prior art to use pivotal linkages between the towing and towed vehicles. One such connection is shown in U.S. Pat. No. 4,019,754 to Hinckley. However, such prior art devices merely create an apparent vertical axis for pivotal movement forward of the linkages in an effort to simulate a hitch connection at that location. Pivotal movement around that location can cause the trailer to cut inside the path of the towing vehicle when turning. The turning radius is also limited by the angle of pivot available behind a towing vehicle using such a hitch mechanism.
Another deficiency with prior art devices, even the fifth wheel connection, is that every lateral force, both large and small, is transmitted directly from one vehicle to the other. Small maneuvers of either vehicle at highway speeds can therefore initiate oscillations in the other vehicle which may result in a resonating interaction making the combination unstable. It is therefore highly desirable to prevent lateral forces from being transferred between vehicles at highway speeds.
A further problem with prior art trailer steering devices is that relative movement between the two vehicles about the pitch axis introduces an undesirable turning input to the wheels through the steering linkages.
The prior art has also employed positive caster to stabilize steerable wheels both on towing and towed vehicles. Positive caster of the trailer wheels in the forward direction produces negative caster in the rearward direction. Negative caster during backing of a split-axle trailer can result in adverse steering inputs which are difficult to control and may cross up the trailer steering assembly. If backing is continued after such cross up, the steering assembly can be severely damaged.